This study was originated because of 4 patients who were found, unexpectedly, to have endogenous immunoglobulins which bound to mouse IgG, resulting in factitiously high serum TSH concentrations using a two-site sandwich IRMA based on mouse monoclonal antibodies. This phenomenon was studied and described to an extent greater than previously reported. In the current year 48 patients who had received radiolabeled mouse monoclonal antibodies for imaging or therapy were studied because these patients sometimes become immunized and have circulating human antimouse-monoclonal antibodies (HAMA). At least 18 patients had evidence of HAMA capable of interfering in an IRMA for TSH. However, steps taken by the manufacturer to suppress this interference appeared to be adequate.